1. Field of the Invention
This invention relates generally to RF amplifiers. In particular, the present invention relates to NMR and MRI pulse amplifiers where fast blanking speed, low blanked noise and a low power mode are desirable, if not mandatory performance features.
2. Description of the Prior Art.
A block diagram of a conventional NMR/MRI pulse amplifier is shown in FIG. 1. The prior art employs a high gain Community Antenna Television (CATV) amplifier in the first stage 10, followed by two low gain middle and output stages 12, 14. Operation of the amplifier in the specific areas of interest is as follows:
i. Fast Blanking Speed--To achieve signal blanking, the bias of both the middle and output stages is turned off. The first stage cannot be turned off because of the CATV design. The existing circuit provides a marginally acceptable blanking speed of slightly more than 1.0 .mu.s. PA1 ii. Low Blanked Noise--When the bias of the output stage is turned off, the residual noise from the NMR/MRI amplifier is about 25 dB above thermal noise. This occurs in existing amplifier designs where the middle stage is left on for other reasons. When the bias of both the middle and output stages is turned off, the residual noise of the NMR/MRI amplifier is about 5-10 dB above thermal noise. PA1 iii. Low Power Mode--In its current configuration, the prior art cannot provide a low power mode. To reduce the output power, the input signal level can be attenuated but the output noise will remain the same. Such a condition is undesirable because the signal-to-noise ratio would deteriorate as the signal level is reduced.
An alternative approach would be to keep the input signal constant while the output signal is attenuated. This would achieve the intended purpose of reducing both the output signal and the noise floor, but this requires the use of a high power switchable attenuator which, because of the high power dissipation requirement, is impractical.
The present invention overcomes these inherent limitations of the prior art.